A variety of processes and techniques have been developed to separate and recover helium from multi-component gas streams.
Such processes include stand-alone membrane units, stand-alone cryogenic units and combinations of membrane units, cryogenic units and pressure swing adsorption (PSA) units. Stand-alone cryogenic processes have been used to produce crude helium at high recovery from natural gas or other streams containing low purity helium. When the concentration of helium in the feed drops to low levels; e.g., below about 1%, processes using stand-alone cryogenic units become inefficient and impractical.
Processes utilizing stand-alone semi-permeable membrane units have the disadvantage in that the helium is subjected to considerable pressure loss when permeating through the membrane making the system energetically unfavorable, especially since the helium product is generally needed at high pressure. Additionally, in order to achieve high helium recoveries and/or purities, staging several membrane units is required, thereby greatly increasing the energy requirements.
U.S. Pat. No. 3,250,080 discloses a process for fractionating a gas mixture containing first and second components having different diffusion rates. The gas mixture is passed through a series of diffusion cells having high and low pressure sides separated by semi-permeable membranes. A product stream of diffused gas enriched in the more diffusible component is withdrawn from the low pressure side, and similarly residual gas enriched in the less diffusible component is withdrawn from the high pressure side in the series. A portion of the diffused gas product may be subjected to low temperature liquefaction and fractionation to obtain a fraction further enriched in the desired component of the gaseous mixture.
U.S. Pat. No. 3,324,626 discloses a process for recovering helium from a gaseous mixture containing less than about 25% helium. The process involves passing the helium-containing gaseous mixture through a permeation stage comprising a high pressure side and a low pressure side separated by a thin, non-porous film having selective permeability for helium. The permeate helium stream is subsequently compressed and cooled to liquefy and separate non-helium constituents.